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Abstract

Though mule deer (Odocoileus hemionus) persist in robust populations throughout most of their North American distribution, nearly 60% of their historic range in México has declined due to habitat loss and unregulated hunting. Two of the six subspecies inhabiting México's deserts and Baja California peninsula are of conservation concern, occurring on land bridge islands in the Pacific Ocean (O. h. cerrosensis on Cedros Island: threatened) and in the Sea of Cortés (O. h. sheldoni on Tiburón Island: endangered). Focusing on the desert southwest (n=449 deer), we obtained 1,611 bp of mtDNA sequence (control region: 583 bp; cytochrome b gene: 1,028 bp) from natural history specimens of Tiburón (n=14) and Cedros (n=15) deer from North American collections to complete the phylogeographic evaluation of the species complex. We found that both island subspecies nest phylogenetically within mainland lineages but demonstrate significantly reduced genetic variation (haplotype diversity for Cedros: p<0.0045; for Tiburón: p<0.0001) compared to their adjacent mainland counterparts. Tiburón deer form the western periphery of an unexpected geographic discontinuity for one of the six inferred mule deer mitochondrial lineages in the desert southwest which is supported as an older, basal lineage by Bayesian phylogenetic inference and relative divergence time estimates. Considering genetic signature of demographic expansions coincide with the full extent and retreat of the last glacial maximum (LGM) (estimated 7,000-26,500 years before present), we propose that the associated climatic fluctuations and drastic turnover in biotic communities (large land herbivore extinctions and increasing aridity following recession of mesic forests to higher elevations) greatly contributed to mule deer expansion and ecological adaptation in the desert southwest. This study underscores the role natural history specimens represent for genetic studies of declining or rare populations, allowing us to provide the first phylogeographic analysis of insular mule deer for the region. Considering both Tiburón and Cedros island subspecies lack comprehensive demographic and ecological studies, our phylogeographic inference will help serve future conservation priorities of desert southwest deer for informed management.